It is a good question, right? In English one says “if”, and in Danish one either says “om” or “hvis”.
These are smaller words, and one might think that they are not so important; that it does not matter whether you say one or the other.
But the wrong use of these words can create whole new expressions with a whole new meaning.
They are the type of words we call “conjunctions”. Conjunctions are a small group of words, which we use to create connections between sentences. We use them when we have become better at Danish and wish to express ourselves deeper and more advanced.
We have about 15 different conjunctions, and now you will get to know two of them:
OM
You use this one when you are asking a question.
“Ved du, om Peter og Anne kommer med til festen?” =
Do you know if Peter and Anne will join the party?
“Ved du, om vi har fri på mandag?” =
Do you know if we have a day off on Monday?
“Jeg ved ikke, om jeg elsker ham” =
I do not know if I love him.
HVIS
You use this when expressing the conditions of an action.
“Du består din prøve, hvis du øver dig” =
You will pass your test if you practice.
“Jeg er glad, hvis du er glad” =
I am happy if you are happy.

Dear Mette,
I had my own translation of the word “hvis”, that is: only if. Maybe it is not true but it helped me to use it in the correct way.
Best regards,
Your student
How nice!
I like that.
It was great. I understand perfectly and can differentiate between two words.